Citrea's Clementine Bridge marks the first time that a fully programmable layer is tested for settling on Bitcoin.

GEORGE TOWN, Cayman Islands, April 21, 2025 /PRNewswire/ -- Citrea, the first ZK rollup to enhance capabilities of Bitcoin blockspace and enable Bitcoin applications (₿apps), has successfully deployed its Clementine Bridge, the first complete BitVM bridge design on Bitcoin Testnet. Citrea has also recently revealed Clementine's whitepaper, marking the first real-world implementation of a BitVM-based bridge which extends BTC's utility to decentralized finance in the most Bitcoin-secured and native way possible. With Clementine's testnet deployment, Bitcoin is for the first time tested for optimistically verifying a fully programmable layer. The Clementine whitepaper is currently the only documentation of how to use BitVM in a bridge design with clear solutions toward a secure and efficient bridge implementation. 

Despite being the world's largest digital asset, BTC has struggled to build relevance in decentralized finance. A secure bridge between Bitcoin and a secondary layer has always been a bottleneck for using BTC in a programmable environment. This problem has forced users to either keep Bitcoin as a passive store of value or rely on custodians for using BTC in financial activities. Citrea's BitVM-based Bitcoin bridge Clementine solves this by enabling the safest and most trust-minimized way to bridge BTC and extend its utility to decentralized finance. 

Citrea's Clementine bridge marks a historic moment for Bitcoin and a new beginning for BTC's utility. "It felt like magic when we saw all the bridge components work smoothly in our internal tests and that any malicious act is stopped by a single honest actor," said Ekrem Bal, Co-Creator of Citrea. "We're now ready to test Clementine in a real Bitcoin Testnet environment and make it more battle-tested for Citrea mainnet."

With the introduction of BitVM by a Bitcoin developer Robin Linus, verifying computations on Bitcoin and building Bitcoin-secured bridges became a possibility. Very soon after its introduction, Citrea team became a core contributor of BitVM and later a founding member of BitVM alliance, where BitVM's fundamental technology of executing a fraud proof of a zero-knowledge (ZK) proof on Bitcoin was developed collaboratively. Although accomplishing a ZK proof verification on Bitcoin was a prerequisite, there were still critical problems to address to implement this fundamental technology securely and efficiently into a Bitcoin bridge design. A detailed breakdown of this can be found in Clementine's whitepaper.

Clementine's design and whitepaper introduce novel solutions to two main research topics in BitVM-based bridge implementation, namely Bitcoin chain fork selection and capital efficiency of operators. These topics have been critical to solve as they ensure both the bridge's security and efficiency. Citrea's Clementine bridge offers clear solutions to both of these problems:

1. Bitcoin Chain Fork Selection: This is related to ensuring that the bridge follows the longest Bitcoin chain. It has been an open research topic where the BitVM creator introduced superblocks approach but its security was later found to be broken. Citrea introduces a new Bitcoin Light Client design that uses succinct Header Chain Proofs of Bitcoin. These proofs are aggregated by operators into a single BitVM proof. 
2. Capital Efficiency of Operators: This has been another limitation of implementing BitVM efficiently into a bridge design. With Citrea's approach to non-standard Bitcoin transactions, operators can now process hundreds of withdrawals in parallel with a single collateral. Disproving only one withdrawal is enough to stop other malicious withdrawal processes.

Clementine's design relies on zero-knowledge proofs, Bitcoin data availability, a novel Bitcoin Light Client design and BitVM. In the case of a malicious act, an honest watchtower provides a proof of the longest Bitcoin chain. Then the operator generates a proof verifying Citrea's state, watchtowers' challenges and payout for the withdrawal. If the operator is malicious, it won't be able to generate such proof, resulting in slashing the operator. Clementine has three main bridge actors to ensure the system's integrity:

• Signers enforce BitVM spendings by managing peg-ins through presign transactions
• Watchtowers are tasked with monitoring operators and challenging any malicious behavior by sending a proof of longest Bitcoin chain
• Operators, a group of profit-seeking entities, temporarily cover users' withdrawals and are later reimbursed with presigned transactions.

Citrea: Bitcoin's Application (₿app) Layer

Citrea extends Bitcoin's utility without sacrificing its security. With zero-knowledge technology, Citrea enables Bitcoin to function as a fully programmable layer. With its Bitcoin bridge Clementine, Citrea offers the safest and most trust-minimized way to bridge BTC to a fully programmable secondary layer. 

For the first time in its history, Bitcoin Network will be able to support diverse on-chain applications and its utility will be extended to decentralized finance. 

Apply to Citrea's unique incubation program 'Citrea Origins' to build ₿apps.

For more information, please visit: Citrea website | Citrea X Account